1. Field of the Invention
The present invention relates to automated devices for making dental appliances, and particularly to a system for vacuum formation of a dental appliance.
2. Description of the Related Art
FIG. 1 illustrates a typical dental retainer R, including a retainer body 12, which is vacuum formed from a sheet or plate of thermoplastic material 20 (as will be described in detail below with regard to FIG. 4) on a dental impression cast 16 (shown in FIG. 3). The dental impression cast 16 is formed from a conventional dental impression tray 10, shown in FIG. 1, using a conventional vacuum thermoforming machine 32 (shown in FIG. 3. The retainer body 12 is characterized by multiple tooth impressions 17, corresponding to the respective teeth of the patient's lower anterior dentition. The retainer body 12 includes a lingual surface 13, which covers the lingual gingiva 19 of the dentition during use of the retainer, and a facial or labial surface 48, which covers the labial surface of the dentition.
Projecting divots 15 are typically formed in the labial or lingual surface of the retainer body 12 and extend into the plastic tooth impressions 17, corresponding to teeth that are in need of repositioning. Openings or windows 46 are cut in the retainer body 12 on the opposite labial or lingual side from the respective divots 15 to accommodate repositioning movement of the teeth in the tooth socket, since the divots 15 apply constant repositioning pressure to the respective teeth 14 while the retainer R is worn on the dentition over a period of time. The retainer body 12 is maintained in position on the patient's dentition by snugly engaging the natural undercuts below the contact points of adjacent teeth, and may be removed as desired.
Retainer R is constructed by first making a dental impression 43 of the patient's upper or lower dentition, or both, preferably using a precision impression material, such as polyvinyl siloxane, and a standard or conventional dental impression tray 10, as shown in FIG. 2. A dental impression cast 16 is then made from that portion of the dental impression 43 corresponding to the area of malpositioned teeth (often the anterior dentition, or the portion of the dentition extending from the left canine to the right canine, as illustrated in FIG. 3). The dental impression cast 16 includes a lingual surface 47, a labial surface and cast teeth 18.
Prior to forming the retainer R, the dental impression cast 16 typically is dried thoroughly and trimmed so that the occlusal surfaces 49 of the cast teeth 18 have a slanted or tapered configuration, as shown in FIG. 3, to facilitate easy removal of the retainer body 12 from the dental impression cast 16. Because the retainer R is maintained in position on the patient's dentition by “snapping into” the multiple undercuts below the contact points of adjacent teeth, the undercuts on the dental impression cast 16 may need trimming for augmentation if their presence on the dental impression cast 16 is not evident.
FIG. 4 illustrates a typical prior art vacuum thermoforming machine 32, having a base 33 with a perforated top vacuum plate and a heating unit 34 extending from the base 33 and mounted on a frame post 41. Heating unit 34 is energized by a heater switch 37, and the machine 32 is used to vacuum thermoform a retainer body 12 having tooth impressions 17 matching the cast teeth 18 of the dental impression cast 16.
The vacuum thermoforming machine 32 also includes a slidable frame 36 having a top frame member 36a hinged to a bottom frame member 36b. The top frame member 36a is removably latched to the bottom frame member 36b by means of a frame latch knob 39. A vacuum motor is contained in the base 33 and is energized by a vacuum motor switch 38.
The retainer body 12 is formed on the dental impression cast 16 by first energizing the heating unit 34 of the vacuum thermoforming machine 32 by actuation of the heater switch 37. The dental impression cast 16 is then placed on the perforated vacuum plate on the top of the base 33, with the cast teeth 18 of the dental impression cast 16 facing upwardly. Before the frame 36 is raised on the frame post 41 by means of frame lift knobs 40 to within a suitable heating distance of the heating unit 34, the top frame member 36a is pivoted upwardly with respect to the bottom frame member 36b. 
A thermoplastic plate 20 is then centered on the bottom frame member 36b. The top frame member 36a is then pivoted downward and secured by the frame latch knob 39, and the frame 36 is raised on the frame post 41 so that the thermoplastic plate 20 is located immediately beneath the heating unit 34. After approximately twenty-five to fifty seconds, the thermoplastic plate 20 is heated to a suitable thermoforming temperature and typically begins to sag slightly, but should not be heated to such a temperature that it is allowed to sag about half an inch or more.
The vacuum motor in the base 33 is then energized by actuation of the vacuum motor switch 38, and the frame 36 is rapidly lowered on the frame post 41 over the vacuum plate of the base 33 by the frame lift knobs 40, so that the softened thermoplastic plate 20 is first draped and then tightly vacuum-pulled over the dental impression cast 16. After ten to fifteen seconds, the retainer body 12 has been formed from the thermoplastic plate 20, and the heating unit 34 is turned off. Immediately after thermoforming the retainer body 12 on the dental impression cast 16, the retentive proximal undercuts in the retainer body 12 can be enhanced, as needed, while the plastic retainer body 12 remains formable.
As the retainer body 12 begins to set and cool on the dental impression cast 16, the retainer body 12 contracts against the dental impression cast 16. This thermal contraction of the retainer body 12 causes the impressionable plastic of the retainer body 12 to conform to the configuration and texture of the dental impression cast 16, including that portion of the dental impression cast 16 corresponding to the natural undercuts of the patient's dentition. The retainer body 12 is then removed from the cast 16 for insertion in the patient's mouth.
In the prior art system 32, the dentist or dental technician is required to manually lower the frame 36 over the cast 16 and determine by sight alone when the plastic plate 20 has achieved a desired, melted thickness. This can lead to inaccuracies in the thickness and overall configuration of the finished retainer R, as there is no standardization or regulation of the melting time or thickness for plate 20. Thus, a system for vacuum formation of a dental appliance solving the aforementioned problems is desired.